Further segmentation efficiency improvements

[CPBridge]

Several important changes to improve efficiency of segmentation creation, including:

• Avoiding unnecessary image resize operations in highdicom.seg.pyramid.create_segmentation_pyramid
• Avoiding unnecessary construction of plane_positions when creating TILED_FULL segmentations
• Avoiding unnecessary construction of source_plane_positions when creating segmentations under certain circumstances
• Incrementally encoding frames of the pixel array when creating native encoded binary segmentations with bitpacking. This saves a lot of memory in some situations and saves a large allocation when the frames are concatenated at the end.
• Using lower level pydicom functionality to avoid keyword dictionary lookups in key functions that are called in loops, in particular Segmentation._get_pffg_item and constructor of PlanePositionSequence